The food industry has recently focused attention on polyol polyesters for use as low-calorie fats in food products. Triglycerides (triacylglycerols) constitute about 90% of the total fat consumed in the average diet. One method by which the caloric value of edible fat can be lowered is to decrease the amount of triglycerides that are consumed, since the usual edible triglyceride fats are almost completely absorbed in the human system (see Lipids, 2, H. J. Deuel, Interscience Publishers, Inc., New York, 1955, page 215). Low calorie fats which can replace triglycerides are described in Mattson, et al., U.S. Pat. No. 3,600,186. Mattson, et al. disclose low calorie, fat-containing food compositions in which at least a portion of the triglyceride content is replaced with a polyol fatty acid ester having at least four fatty acid ester groups, with each fatty acid having from eight to twenty-two carbon atoms.
A number of process have been disclosed in the art for preparing highly esterified polyol fatty acid polyesters, in particular sucrose polyesters. One such process for preparing these polyesters involves a solvent-free, essentially two-step transesterification of the polyol with fatty acid esters of an easily removable lower alkyl alcohol. In the first step, a mixture of polyol, fatty acid lower alkyl esters, alkali metal fatty acid soap and a basic esterification catalyst are heated to form a melt. The amount of fatty acid lower alkyl esters is such that the melt forms primarily partial fatty acid esters of the polyol, e.g. esters in which less than about 50% of the hydroxyl groups of the polyol are esterified. In the second step, an excess of fatty acid lower alkyl esters is added to the melt which is then heated to convert the partial polyol polyesters to more highly esterified polyol polyesters, e.g. those in which more than 50% of the hydroxyl groups of the polyol are esterified. See, for example, Rizzi & Taylor, U.S. Pat. No. 3,963,399, and Volpenhein, U.S. Pat. No. 4,517,360 and U.S. Pat. No. 4,512,772.
The lower alkyl esters which are used to prepare the polyol polyesters can be prepared by the transesterification of fatty acid sources such as triglyceride oils and fats with a lower alkyl alcohol in the presence of an alkali catalyst. After the transesterification reaction, a crude glycerine-containing layer comprising glycerine (glycerol) formed in the transesterification reaction, catalyst, soap formed by the catalyst, lower alkyl esters and lower alkyl alcohol, is separated from the fatty acid lower alkyl ester layer. The fatty acid lower alkyl ester layer is then purified by any suitable recovery method, such as, e.g., distillation. Processes of this type have been described in U.S. Pat. Nos. 2,383,579, 2,383,580, 2,383,596, 2,383,599, 2,383,601, 2,383,602, 2,383,614, 2,383,632 and 2,383,633, and in the European Patent No. 0 164 643. An extra esterification step before recovery, but after separation of the fatty acid lower alkyl ester layer from the glycerol layer, may also be used to produce high yields of high purity fatty acid lower alkyl esters. See European Patent No. 391 485.
Unfortunately, the lower alkyl esters prepared by any of these known processes are likely to contain some residual level of fat sources such as glycerine, and mono-, di-, or triglyceride. When these lower alkyl esters are then used to prepare polyol fatty acid polyesters, they will cause the polyol polyester product to contain undesirably high levels of triglyceride fat. These triglycerides add calories to the polyol polyesters and keep the polyol polyesters from being completely fat free.
Another disadvantage with known processes for preparing methyl esters is that fatty acid methyl esters with differing lengths of fatty acid chains are not separated from one another. As unsaturated C18 fatty acid esters are particularly suitable for making liquid polyol fatty acid polyesters, while C22 fatty acid esters are particularly suitable for making solid polyol fatty acid polyesters, it would be desirable to separate the fatty acid lower alkyl esters into fractions of specific fatty acid chain lengths.
Another disadvantage with known processes for preparing fatty acid lower alkyl esters is that during distillation undesirably high levels of free fatty acids can be formed, causing the fatty acid lower alkyl esters to have undesirably high acid values (greater than 1.0). This is particularly problematic when distilling fatty acid lower alkyl esters having long chain fatty acid moieties (fatty acid chains of 16 or more carbon atoms). Since the boiling point of a fatty acid lower alkyl ester tends to increase as the fatty acid chain length increases, the distillation temperature generally must also increase accordingly; unfortunately, higher acid values tend to be generated at higher temperatures. Therefore, the longer the fatty acid chain length, the more free fatty acids are likely to be formed during the distillation. Consequently, as the fatty acid chain length increases it becomes increasingly difficult to make fatty acid lower alkyl esters having low acid values (no greater than about 1.0).
Thus, many prior art methods which produce fatty acid lower alkyl esters are limited in that significant levels of glycerine and mono-, di- or triglycerides are contained in the esters whereby the product is not completely fat free. Additionally, many prior art methods are limited in that there is no fractionation between fatty acid lower alkyl esters with varying fatty acid chain lengths. Prior art methods are also limited in that the fatty acid lower alkyl esters generally have high acid values.